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Diltiazem (Cardizem)
Revised: February 10, 2021
Published: June 15, 2012

(For veterinary information only)

The size of the tablet/medication is NOT an indication of a proper dose. Never administer any drug without your veterinarian's input. Serious side effects or death can occur if you use drugs on your pet without your veterinarian's advice. 

It is our policy not to give dosing information over the internet.

Available in 30 mg, 60 mg, 90 mg, 120 mg, tablets; or 120 mg, 180 mg, 240 mg, 300 mg, 360 mg, 420 mg extended-release capsules/tablets

Brand name: Cardizem, Dilacor XR 


Muscle contraction is governed by the flow of calcium through what are called voltage-gated calcium channels. When these channels are blocked by drugs, less calcium can flow through the channels. This results in a weaker muscle contraction. Blood calcium levels are not altered by this blockade.

The voltage-gated calcium channels that one might wish to block are located in heart muscle and in blood vessels. The use of calcium channel blockers would lead to a more relaxed and, therefore, dilated blood. The voltage-gated calcium channels that one might wish to block are located in heart muscle and in blood vessels. In the case of blood vessels, the use of a calcium channel blocker would lead to less muscle tone in the blood vessel. This creates a more relaxed and, therefore, dilated blood vessel that, in turn, translates to lower blood pressure. The heart ends up with less resistance to pump against, meaning that the heart does not have to work as hard. In situations where the heart muscle might be overworked, this might lead to a more efficient and healthier heart.

Diltiazem also affects the electrical conduction of the heart such that electrical conduction is slowed. This means that the heart rate also slows, allowing for a longer filling time for the heart between pumps.

How this Medication is Used

Diltiazem is most commonly used in the treatment of hypertrophic cardiomyopathy in cats. In this condition, the heart muscle becomes stiff and inflexible, plus the filling chambers of the heart become smaller. The muscle-bound heart cannot fill normally for these reasons, and since it cannot fill normally it sends forth less blood with each pump. To meet the body's oxygen demands, the diseased heart must pump faster than normal since it is pumping out less blood per pump. This overworks the heart and can lead to muscle fatigue and heart failure.  By using a calcium channel blocker such as diltiazem, the heart relaxes and slows and thus may fill more efficiently. Better heart chamber filling means more blood output, no need to pump faster, and less heart muscle fatigue.

Diltiazem might also be used to treat high blood pressure (hypertension) should it arise from any of numerous situations.

  • Diltiazem can be given with or without food.
  • Diltiazem is typically given 2-3 times daily. Be sure you understand the dosing instructions.
  • Extended-relief capsules have time-releasing beads. These should not be crushed.
  • If a dose of dilitazem is accidentally skipped, do not double up on the next dose. Simply give the next dose as would normally be scheduled.

Side Effects

The most common side effects are vomiting (generally eased by giving the medication with food). Weight loss may occur after two to six months.

Side effects of serious significance include collapse (if heart rate and/or blood pressure drops too low), weakness, jaundice, and skin rashes. These reactions need medical attention.

Interactions with Other Drugs

Diltiazem should not be used with beta-blockers (heart medications that might compound the low blood pressure side effect), such as atenolol or propranolol. Even beta-blocker eyedrops as might be used in glaucoma treatment, could create adverse blood pressure reactions.

Cimetidine (an antacid) may increase the blood levels of diltiazem (making the effects of diltiazem stronger). Ranitidine, another antacid, also has this effect but not as strongly.

Medications that are stronger when used with diltiazem include fentanyl, hydrocodone, vincristine, benzodiazepine tranquilizers (such as diazepam and alprazolam), and cyclosporine (an immunomodulator used to treat immune-mediated disease as well as allergies).

Medications that are weaker when used with diltiazem include buspirone (an anti-anxiety medication).

Concurrent use of diltiazem with cisapride (a bowel motility modifier most commonly used to treat obstipation/severe constipation) could lead to cardiac arrhythmia (disturbances in the electrical conduction of the heart).

Clopidogrel, a blood thinner, may not work as well in the presence of diltiazem.

Theophylline, an airway dilator, may have more potential for toxicity when used concurrently with diltiazem.

Concurrent use of diltiazem with macrolide class antibiotics (erythromycin, clarithromycin, azithromycin) may increase the toxicity of diltiazem.

Concerns and Cautions

  • Diltiazem should not be used in conditions where reduced strength of heart contraction or further slowing the heart rate could be dangerous. Such situations might include congestive heart failure, AV blockage-type heart arrhythmias, or low blood pressure. 
  • Diltiazem should be avoided in patients with liver impairment or kidney failure. If use cannot be avoided, dose changes will likely be needed. 
  • Diltiazem should not be used during pregnancy. 
  • Diltiazem should be stored at room temperature and protected from light.
  • If you miss a dose, do not double up on the next dose. Simply give the medication when it is remembered or pick up with the next dose, allowing at least the proper interval between doses according to the label instructions.

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